Bi-directional bipolar junction transistor for high voltage electrostatic discharge protection

ABSTRACT

A bi-directional electrostatic discharge (ESD) protection device may include a substrate, an N+ doped buried layer, an N-type well region and two P-type well regions. The N+ doped buried layer may be disposed proximate to the substrate. The N-type well region may encompass the two P-type well regions such that a portion of the N-type well region is interposed between the two P-type well regions. The P-type well regions may be disposed proximate to the N+ doped buried layer and comprise one or more N+ doped plates and one or more P+ doped plates.

TECHNOLOGICAL FIELD

Embodiments of the present invention generally relate to semiconductordevices and, more particularly, relate to a bi-directional bipolarjunction transistor (BJT) for high voltage electrostatic discharge (ESD)protection.

BACKGROUND

There is currently an ongoing drive toward the downscaling of devicedimensions in virtually all aspects of electronic device manufacture.Smaller electronic devices tend to be more popular than larger, morebulky devices when both devices have substantially equivalentcapabilities. Accordingly, being able to fabricate smaller componentswould clearly tend to facilitate the production of smaller devices thatincorporate those components. However, many modern electronic devicesrequire electronic circuitry to perform both actuation functions (e.g.,switching devices) and data processing or other decision makingfunctions. The use of low voltage complementarymetal-oxide-semiconductor (CMOS) technologies for these dual functionsmay not always be practical. Thus, high voltage (or high power) deviceshave also been developed to handle many applications where low voltageoperation is not practical.

The electrostatic discharge (ESD) performance of typical high voltagedevices often depends on the total width and surface or lateral rules ofthe corresponding devices. Thus, ESD performance may typically be morecritical for smaller devices. High voltage devices typically havecharacteristics that include a low on-state resistance (Rdson), a highbreakdown voltage and a low holding voltage. The low on-state resistancemay tend to make an ESD current more likely to concentrate on thesurface or the drain edge of a device during an ESD event. High currentand high electric fields may cause the physical destruction at a surfacejunction region of such a device. Based on the typical requirement for alow on-state resistance, the surface or lateral rules likely cannot beincreased. Thus, ESD protection may be a challenge.

The high breakdown voltage characteristic of high voltage devicestypically means that the breakdown voltage is higher than the operatingvoltage, and the trigger voltage (Vt1) is higher than the breakdownvoltage. Accordingly, during an ESD event, the internal circuitry of thehigh voltage device may be at risk of damage before the high voltagedevice turns on for ESD protection. The low holding voltagecharacteristic of high voltage devices also leaves open the possibilitythat unwanted noise associated with a power-on peak voltage or a surgevoltage may be triggered or that a latch-up may occur during normaloperation. High voltage devices may also experience the field plateeffect due to the fact that electric field distribution may be sensitiveto routing so that ESD current may be likely to concentrate at thesurface or drain edge during an ESD event.

To improve high voltage device performance with respect to ESD events,one technique that has been implemented involves the additional use ofmasks and other processes to create a larger sized diode within bipolarjunction transistor (BJT) components and/or increasing the surface orlateral rules for MOS transistors. Silicone controlled rectifiers (SCRs)have also been developed to protect circuitry during ESD events.However, while the low holding voltage of SCRs means they may performwell during ESD events, this characteristic also increases theoccurrence of latch-up during normal operation.

Motor driver circuits may be particularly troublesome to protect fromESD events using current solutions. This is because when a motor isswitched off, it may continue spinning for some time, thus acting as aninductor which feeds back a high negative voltage. If the motor drivercircuitry were to include a PMOS, the parasitic forward bias diode ofthe PMOS may be turned on by this negative feedback voltage, potentiallycausing latch-up issues and/or other irregular circuit operation.

Accordingly, it may be desirable to develop an improved structure forproviding ESD protection and, in particular, for providingbi-directional ESD protection.

BRIEF SUMMARY OF EXEMPLARY EMBODIMENTS

Some example embodiments are therefore directed to a low voltagestructure bi-directional bipolar junction transistor (BJT) for highvoltage electrostatic discharge (ESD) protection. In some cases, the ESDprotection may be provided at least in part based on modifications to aBCD (Bipolar Complimentary metal-oxide semiconductor (BiCMOS) Diffusionmetal-oxide semiconductor (DMOS)) process that may involve an epitaxialprocess.

In one exemplary embodiment, a bi-directional BJT is provided(“exemplary” as used herein referring to “serving as an example,instance or illustration”). The bi-directional BJT may include a p-typesubstrate, an N+ doped buried layer, an N-type well region and twoP-type well regions. The N+ doped buried layer may be disposed adjacentto the substrate. The N-type well region may be disposed adjacent to theN+ doped buried layer and encompassing the first and second P-type wellregions such that a portion of the N-type well region is interposedbetween the first and second P-type well regions. The P-type wellregions may be disposed adjacent to the N+ doped buried layer and eachmay respectively comprise one or more N+ doped plates and one or more P+doped plates.

According to a further embodiment, the P-type well regions comprisethree N+ doped plates, two P+ doped plates, and two gate structures. Foreach P-type well, the three N+ doped plates, two P+ doped plates, andtwo gate structures may be configured such that a first P+ doped plateis disposed adjacent to a first N+ doped plate, a first gate structureis interposed between the first and a second N+ doped plate, a secondgate structure is interposed between the second and a third N+ dopedplate, and a second P+ doped plate is disposed adjacent to the third N+doped plate.

In another exemplary embodiment, a circuit is provided which comprises abi-directional high voltage ESD protection element. The bi-directionalhigh voltage ESD protection element comprises a p-type substrate, an N+doped buried layer, an N-type well region and two P-type well regions.The N+ doped buried layer may be disposed adjacent to the substrate. TheN-type well region may be disposed adjacent to the N+ doped buried layerand encompassing the first and second P-type well regions such that aportion of the N-type well region is interposed between the first andsecond P-type well regions. The P-type well regions may be disposedadjacent to the N+ doped buried layer and each may respectively compriseone or more N+ doped plates and one or more P+ doped plates. The P-typewell regions may comprise three N+ doped plates, two P+ doped plates,and two gate structures. For each P-type well, the three N+ dopedplates, two P+ doped plates, and two gate structures may be configuredsuch that a first P+ doped plate is disposed adjacent to a first N+doped plate, a first gate structure is interposed between the first anda second N+ doped plate, a second gate structure is interposed betweenthe second and a third N+ doped plate, and a second P+ doped plate isdisposed adjacent to the third N+ doped plate.

According to yet another exemplary embodiment, a semiconductor device isprovided which comprises a first isolated low voltage n-channel metaloxide field effect transistor (LVNMOS) and a second isolated LVNMOS, thefirst and second isolated LVNMOS sharing a common N-type well isolationregion.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIGS. 1 a and 1 b respectively illustrate a simplified diagram of aprior art SCR and its associated electrical characteristics;

FIGS. 2 a and 2 b respectively illustrate a simplified diagram of anembodiment of the present invention and its associated electricalcharacteristics;

FIGS. 3 a and 3 b illustrate electrical circuits having electricalproperties roughly equivalent to an embodiment of the present invention;

FIGS. 4 a and 4 b illustrate the circuit representations depicted inFIGS. 2 a and 2 b under positive ESD stress;

FIGS. 5 a and 5 b illustrate the circuit representations depicted inFIGS. 2 a and 2 b under negative ESD stress;

FIG. 6 illustrates a cross-sectional view of the structure of an exampleembodiment;

FIG. 7 illustrates a cross-sectional view of the structure of an exampleembodiment under positive ESD stress;

FIG. 8 illustrates a cross-sectional view of the structure of an exampleembodiment under negative ESD stress;

FIG. 9 illustrates breakdown voltage characteristics and experimentalelectrical characteristics of an example embodiment.

DETAILED DESCRIPTION

Some example embodiments of the present invention will now be describedmore fully hereinafter with reference to the accompanying drawings, inwhich some, but not all embodiments of the invention are shown. Indeed,various example embodiments of the invention may be embodied in manydifferent forms and should not be construed as limited to the exampleembodiments set forth herein; rather, these example embodiments areprovided so that this disclosure will satisfy applicable legalrequirements.

Some example embodiments of the present invention may provide abi-directional BJT that may, for example, be used for bi-directionalhigh voltage ESD protection, e.g., protection for positive and negativevoltage ESD. The bi-directional BJT of example embodiments may combinetwo isolation low voltage N-channel metal oxide semiconductortransistors (MOS) into one ESD protection device, thus providing astructure having a total area smaller than a diode—BJT and MOS, whileproviding similar ESD performance in two directions. Example embodimentsmay also have a breakdown voltage that is near the high voltage deviceoperation voltage and a trigger voltage that is lower than the highvoltage device breakdown voltage. Furthermore, a relatively high holdingvoltage may be provided to more easily avoid latch-up occurrence thanwith a silicon controlled rectifier (SCR). Example embodiments may, forexample, be useful in motor driver circuits, such as connected betweenan I/O pad and power pad. In this case, example embodiments may providepositive and negative high voltage ESD protection without causingirregularities during normal operation and without introducing latch-upissues. Example embodiments may also be fabricated, in some cases, witha standard BCD process that does not require the addition of anincreased number of masks or processes. The polysilicon used in someexample embodiments may, for example, be provided via a hard mask at ionimplantation

FIG. 1 a illustrates a simplified diagram of a conventional SCR 100. Asshown, a conventional SCR consists of a P+ material 101 adjacent to anN− material 102, which is in turn adjacent to a P-type material 103 thatis itself adjacent to an N+ material 104. An electrically-equivalentdiagram 102 is also depicted. As shown in the graph 160 of FIG. 1 b, aconventional SCR provides ESD protection in the forward direction, asillustrated by the snap-back 161 which occurs at the forward breakdownvoltage.

FIG. 2 a illustrates a simplified diagram of an embodiment of thepresent invention. As shown in view 200, embodiments of the presentinvention may operate as two NPN bipolar transistors 201 having coupledN-type regions 202. Thus, as can be seen in views 210 and 220, exampleembodiments may function so as to be triggered by a forward bias diode211, then turn-on an NPN BJT 212 to snap-back, in both forward 210 andreverse 220 directions. The graph 230 illustrates the aforementionedforward and reverse snap-backs 231. Example embodiments may have lowon-resistance (Ron) and high holding voltage, and high ESD current maybe discharged by the forward bias diode and NPN BJT at the same time.

FIGS. 3 a and 3 b illustrate simplified circuit diagram representationsof an embodiment of the present invention. As can be seen in FIG. 3 a,embodiments of the present invention may comprise two low voltageisolation NMOSs 300 a, 300 b sharing a common isolation region 301. Asshown in FIG. 3 b, the electrical properties of embodiments of thepresent invention may be modeled as two BJT transistors 310 a, 310 bwith coupled collectors 311. As can be seen in FIGS. 4 a and 4 b, underpositive ESD stress, the top BJT transistor 310 a instead operates as aforward bias diode 410 a. As can be seen in FIGS. 5 a and 5 b, undernegative ESD stress, the bottom BJT transistor 310 a instead operates asa forward bias diode 510 b. Thus, whether positive ESD or negative ESDstress is applied, embodiments of the present invention may ensure ESDcurrent is discharged, thus providing bi-directional ESD protection. Theforward and reverse breakdown voltages of example embodiments may bemade the same or different by using isolation NMOS or NPN BJTs havingthe same or different breakdown voltage.

Having thus described generally the electrical characteristics andproperties of example embodiments of the present invention, referencewill now be directed to FIGS. 6 through 8 in order to describe thestructure of an example embodiment.

FIG. 6 illustrates a cross-sectional view of an example embodiment forproviding bi-directional high voltage ESD protection. As can be seenfrom FIG. 6, a P-type material substrate 600 or an epitaxially-grownP-layer (P-epi) may be provided with an N+ buried layer 601 disposedadjacent thereto. An N-type well 602 a-c may be disposed adjacent to theN+ buried layer 601 and encompassing first and second P-type wells 603a, 603 b such that a portion 602 b of the N-type well over is disposedbetween the first and second P-type wells 603 a, 603 b. The N-type well602 a-c may be a single contiguous well according to some embodimentsor, according to another embodiment, may comprise two or more separateN-type wells. The outer portions of the N-type well 602 a, 602 c may bein contact with the P-type substrate 600 according to an exampleembodiment. The first and second P-type wells 603 a, 603 b may compriseat least one P+ doped plate 604 and at least one N+ doped plate 605.

For example, according to the example embodiment depicted in FIG. 6, thefirst and second P-type wells 603 a, 603 b may each comprise two P+doped plates 604, three N+ doped plates 605, and two gate structures606. Thus, as shown, the first P-type well 603 a may comprise a first P+doped plate 604 that may be disposed adjacent to a first N+ doped plate605, a first gate structure 606 that may be interposed between the firstand a second N+ doped plate 605, a second gate structure 606 that may beinterposed between the second and a third N+ doped plate 605, and asecond P+ doped plate that may be disposed adjacent to a third N+ dopedplate. Similarly, the second P-type well 603 b may comprise a third P+doped plate 604 that is disposed adjacent to a fourth N+ doped plate605, a third gate structure that is interposed between the fourth and afifth N+ doped plate 605, a fourth gate structure that is interposedbetween the fifth and a sixth N+ doped plate 605, and a fourth P+ dopedplate 604 that is disposed adjacent to the sixth N+ doped plate.According to another example embodiment, an anode 607 may be operablyconnected to the P+ doped plates 604, N+ doped plates 605, and gatestructures 606 of one of the P-type wells 603 a and a cathode 608 may beoperably connected to the P+ doped plates 604, N+ doped plates 605, andgate structures 606 of the other of the P-type wells 603 b.

The gate structures 606 which may be formed between the N+ doped plates605 may include a gate oxide layer and a layer of polysilicon, where thepolysilicon may be provided as a hard mask at ion implantation. The gate606 may enable collective operation of the distributed N+ doped plates605. Field-oxide film (FOX) portions 609 may be disposed adjacent to thesurface of portions of the N-type well 602 a-c and adjacent a distal endof each of the P+ doped plates 604. As can be seen from FIG. 6, multipleBJT transistors 610 a, 610 b (in this example there are four, twoanode-side 610 a and two cathode-side 610 b) may be effectively formedby the provided structure. As shown, the collectors (denoted as “C” inFIG. 6) of the anode-side BJT transistors 610 a and cathode-side BJTtransistors 610 b are effectively connected according to the depictedstructure. Moreover, the bases (denoted as “B” in FIG. 6) of theanode-side BJT transistors 610 a and cathode-side BJT transistors 610 bare effectively connected to their respective P+ plates and the emitters(denoted as “E” in FIG. 6) of the anode-side BJT transistors 610 a andcathode-side BJT transistors 610 b are effectively connected to theirrespective N+ plates.

It will be understood that the configuration depicted in FIG. 6 and,indeed, configurations according to other embodiments which are notdepicted, may function as two isolated low voltage NMOS which share acommon N-type isolation region 301. That is, the substrate 600, the N+buried layer 601, the N-the type well 602 a, 602 b, the P-type well 603a, along with the P+ plates 604, N+ plates 605, and, according to someembodiments, gate structures 606 associated with the P-type well 603 a,may function as a first isolated low voltage NMOS 300 a. Likewise, thesubstrate 600, the N+ buried layer 601, the N-the type well 602 c, 602b, the P-type well 603 b, along with the P+ plates 604, N+ plates 605,and, according to some embodiments, gate structures 606 associated withthe P-type well 603 b, may function as a second isolated low voltageNMOS 300 b. The shared common N-type isolation region 301 thus comprisesN-type well 602 b. The gate, source, and drain of the isolated lowvoltage NMOSs 300 a, 300 b are denoted in FIGS. 6 through 8 as “G,” “S,”and “D,” respectively.

As shown in FIGS. 7 and 8, respectively, during a positive ESD event,the anode-side transistors 610 a may, in effect, operate as forward biasdiodes 710 a, and during a negative ESD event, the cathode-sidetransistors 610 b may, in effect, operate as forward bias diodes 810 b.Thus, during either a positive or negative ESD event, ESD current may bedischarged by a forward bias diode and an NPN BJT at the same time.

The material of the N+ buried layer 601 may be N-epi, a deep N-typewell, or multiple, stacked N+ buried layers. The P-type wells 603 a, 603b may be stacked with a P-type well and P+ buried layer or a P-implant.The N-type wells 602 a-c may also be an N-implant in some cases. Thestructure may be fabricated using any standard BCD process withoutadditional masks. According to another example embodiment, the structuremay be fabricated with a non-epitaxial process, such as a triple wellprocess. The structure may also be fabricated with a single poly ordouble poly process. A local oxidation of silicone (LOCOS) process maybe used in the fabrication of at least a portion of the structure, suchas to fabricate the FOX portions 609. Alternatively, a shallow trenchisolation (STI) process may be used, such as to fabricate at least aportion of the structure, such as the FOX portions 609.

FIG. 9 includes a topmost graph 900 illustrating the breakdown voltagecharacteristics of an example embodiment. As can be seen from the graph900, the breakdown voltage may have an equal magnitude in the forward(positive) and reverse (negative) direction. The bottom graphs 910, 920illustrate measured leakage current 911, 921 between the anode 607 andcathode 608, and measured ESD current 912, 922 of an example embodimentduring positive and negative ESD stress experiments, respectively. Ascan be seen, the both of the measured ESD currents 912, 922 exhibitsnap-back 931, indicating successful ESD protection in both the positiveand negative direction.

Example embodiments may therefore provide a relatively small-sizedbi-directional bipolar junction transistor (BJT) for high voltageelectrostatic discharge (ESD) protection. Moreover, example embodimentsmay be applied to a standard BCD process without a requirement for useof additional masks. Embodiments may also be applied to different highvoltage BCD processes and provide different operational voltage relatedESD protection in the same process by providing a N+ buried layer orN-type well recipe. As such, high voltage ESD protection that is oftenrequired for devices that are to be used in high voltage settings thatmay encounter ESD events can be provided in a relatively small size andby a relatively low voltage MOS structure. Some embodiments could alsobe used for general DC circuit operation. Additionally, ESD protectionmay be provided for devices which require such protection to bebi-directional, such as in motor driver circuits. In this regard,embodiments may, for example, be operably connected between aninput/output (I/O) pad and a power pad of the motor driver circuit so asto provide positive and negative high voltage ESD protection withoutcausing irregular operation or inducing latch-up issues.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Moreover, although the foregoing descriptions and the associateddrawings describe exemplary embodiments in the context of certainexemplary combinations of elements and/or functions, it should beappreciated that different combinations of elements and/or functions maybe provided by alternative embodiments without departing from the scopeof the appended claims. In this regard, for example, differentcombinations of elements and/or functions than those explicitlydescribed above are also contemplated as may be set forth in some of theappended claims. Although specific terms are employed herein, they areused in a generic and descriptive sense only and not for purposes oflimitation.

What is claimed is:
 1. A bi-directional bipolar junction transistor(BJT) comprising: a p-type substrate; an N+ doped buried layer disposedadjacent to the substrate; a first P-type well region disposed adjacentto the N+ doped buried layer; a second P-type well region disposedadjacent to the N+ doped buried layer; and an N-type well regionadjacent to the N+ doped buried layer and encompassing the first andsecond P-type well regions such that at least a portion of the N-typewell region is interposed between the first and second P-type wellregions; wherein the first and second P-type wells each comprise atleast one N+ doped plate and at least one P+ doped plate.
 2. Thebi-directional BJT of claim 1, wherein the first P-type well comprisesfirst, second, and third N+ doped plates, first and second P+ dopedplates, and first and second gate structures, the first P+ doped platebeing disposed adjacent to the first N+ doped plate, the first gatestructure being interposed between the first and second N+ doped plate,the second gate structure being interposed between the second and thirdN+ doped plate, and the second P+ doped plate being disposed adjacent tothe third N+ doped plate; and further wherein the second P-type wellcomprises fourth, fifth, and sixth N+ doped plates, third and fourth P+doped plates, and third and fourth gate structures, the third P+ dopedplate being disposed adjacent to the fourth N+ doped plate, the thirdgate structure being interposed between the fourth and fifth N+ dopedplate, the fourth gate structure being interposed between the fifth andsixth N+ doped plate, and the fourth P+ doped plate being disposedadjacent to the sixth N+ doped plate.
 3. The bi-directional BJT of claim2, further comprising first, second, and third field oxide (FOX)portions disposed adjacent to the N-type well region, the first FOXportion being further disposed adjacent to the first P+ doped plate, thesecond FOX portion being further interposed between the second and thirdP+ doped plate, and the third FOX portion being disposed adjacent to thefourth P+ doped plate.
 4. The bi-directional BJT of claim 3, wherein thefirst, second, and third FOX portions are fabricated via a localoxidation of silicon (LOCOS) process.
 5. The bi-directional BJT of claim3, wherein the first, second, and third FOX portions are fabricated viaa shallow trench isolation (STI) process.
 6. The bi-directional BJT ofclaim 2, wherein the gate structures comprise a polysilicone layer. 7.The bi-directional BJT of claim 6, wherein the polysilicone layer isprovided as a hard mask at ion implantation.
 8. The bi-directional BJTof claim 1, wherein the N+ buried layer comprises an n-type epitaxiallayer.
 9. The bi-directional BJT of claim 1, wherein the N+ buried layercomprises a deep N-type well.
 10. The bi-directional BJT of claim 1,wherein the N+ buried layer comprises a plurality of stacked N+ buriedlayers.
 11. The bi-directional BJT of claim 1, wherein each P-type wellcomprises a stacked P-type well and P+ buried layer.
 12. Thebi-directional BJT of claim 1, wherein the P-type wells are fabricatedvia P-type implantation.
 13. The bi-directional BJT of claim 1, whereinthe N-type well region is fabricated via N-type implantation.
 14. Thebi-directional BJT of claim 1, wherein the bi-directional BJT isfabricated via a single poly process.
 15. The bi-directional BJT ofclaim 1, wherein the bi-directional BJT is fabricated via a double polyprocess.
 16. The bi-directional BJT of claim 1, wherein thebi-directional BJT is fabricated via a non-epitaxial process.
 17. Thebi-directional BJT of claim 16, wherein the non-epitaxial processcomprises a triple-well process.
 18. A circuit comprising abi-directional high voltage electrostatic discharge (ESD) protectionelement, the bi-directional high voltage ESD protection elementcomprising: a p-type substrate; an N+ doped buried layer disposedadjacent to the substrate; a first P-type well region disposed adjacentto the N+ doped buried layer; a second P-type well region disposedadjacent to the N+ doped buried layer; and an N-type well regionadjacent to the N+ doped buried layer and encompassing the first andsecond P-type well regions such that at least a portion of the N-typewell region is interposed between the first and second P-type wellregions; wherein the first P-type well comprises first, second, andthird N+ doped plates, first and second P+ doped plates, and first andsecond gate structures, the first P+ doped plate being disposed adjacentto the first N+ doped plate, the first gate structure being interposedbetween the first and second N+ doped plate, the second gate structurebeing interposed between the second and third N+ doped plate, and thesecond P+ doped plate being disposed adjacent to the third N+ dopedplate; and further wherein the second P-type well comprises fourth,fifth, and sixth N+ doped plates, third and fourth P+ doped plates, andthird and fourth gate structures, the third P+ doped plate beingdisposed adjacent to the fourth N+ doped plate, the third gate structurebeing interposed between the fourth and fifth N+ doped plate, the fourthgate structure being interposed between the fifth and sixth N+ dopedplate, and the fourth P+ doped plate being disposed adjacent to thesixth N+ doped plate.
 19. The circuit of claim 18, wherein thebi-directional high voltage ESD protection element further comprises: ananode operably connected to the first, second, and third N+ dopedplates, the first and second P+ doped plates, and the first and secondgate structures; and a cathode operably connected to the fourth, fifth,and sixth N+ doped plates, the third and fourth P+ doped plates, and thethird and fourth gate structures; further wherein the circuit comprisesa motor driver circuit comprising an input/output (I/O) pad and a powerpad, one of the anode or cathode of the bi-directional high voltage ESDprotection element being operably connected to the I/O pad and the otherof the anode or cathode of the bi-directional high voltage ESDprotection element being operably connected to the power pad.
 20. Asemiconductor device comprising a first isolated low voltage n-channelmetal oxide field effect transistor (LVNMOS) and a second isolatedLVNMOS, wherein the first and second isolated LVNMOS share a commonN-type well isolation region.